1. Technology Field
The present invention relates to a data storing method for a rewritable non-volatile memory module, a memory control circuit unit using the method, and a memory storage apparatus using the method.
2. Description of Related Art
Digital cameras, mobile phones, and MP3 players have undergone rapid growth in recent years, so that consumers' demands for storage media have also increased drastically. Since a rewritable non-volatile memory is characterized by non-volatility of data, low power consumption, small volume, non-mechanical structure, and fast reading and writing speed, the rewritable non-volatile memory is the most adaptable memory to be applied in a portable electronic product, e.g., a notebook computer. A solid state drive (SSD) is a storage apparatus which utilizes a flash memory as its memory storage medium. Therefore, the flash memory industry has become a very popular part of the electronic industry in recent years.
Among the rewritable non-volatile memories, an NAND flash memory is one of the memories which have been extensively used; according to the number of bits which can be stored by each memory cell, the NAND flash memory may be classified into a single level cell (SLC) NAND flash memory, a multi level cell (MLC) NAND flash memory, and a trinary level cell (TLC) NAND flash memory. Specifically, each memory cell in the SLC NAND flash memory stores 1 bit of data (i.e., “1” or “0”), each memory cell in the MLC NAND flash memory stores two bits of data, and each memory cell in the TLC NAND flash memory stores three bits of data.
In the NAND flash memory, a physical programming unit is composed of several memory cells arranged on the same word line. Since each memory cell in the SLC NAND flash memory is capable of storing 1 bit of data, several memory cells arranged on the same word line in the SLC NAND flash memory correspond to one physical programming unit.
By contrast, a floating gate storage layer in each memory cell of the MLC NAND flash memory is capable of storing two bits of data, and each storage state (i.e., “11,” “10,” “01,” and “00”) includes the least significant bit (LSB) and the most significant bit (MSB). For instance, the first bit from the left of the storage states is the LSB, and the second bit from the left of the storage states is the MSB. Accordingly, several memory cells arranged on the same word line may constitute two physical programming units, wherein the physical programming units constituted by the LSB and the MSB of the memory cells are lower physical programming units and upper physical programming units, respectively.
In comparison with the SLC NAND flash memory, the MLC NAND flash memory has larger capacity and costs less, and thus the industry prefers the latter in the memory storage apparatus having the rewritable non-volatile memory module. However, the stability of the MLC NAND flash memory is inferior to that of the SLC NAND flash memory, and thus the bit error count may increase if the number of times of the erasing operations on some physical erasing blocks of the MLC NAND flash memory is insufficient. To prevent loss of data in the physical erasing units with a great number of error bits, once the bit error count is detected to exceed a normal value during a data reading process, the system often moves the data; therefore, the data written into the defective physical erasing units having a great number of error bits are required to be moved again, which compromises the overall performance of the system.